Brotato is possibly the best reverse-bullet-hell game I've played. Our family liked to sit down and pass the controller around playing it, and the latest beta version added an outright co-op mode so we can play together. It is entirely too fun, and simple enough the youngest can play too. All he has to do is move the potato around the screen trying not to get hit, and we can help him with the in between part where you have to read. It is stupid simple fun.

If you like roguelikes, Path of Achra is a very good one focused on tactics and building your character, also made with wokedot.